Various types of medical devices such as cochlear implants, artificial hearts, and neural stimulators have been developed that are designed to be surgically inserted within a patient's body to carry out a medically related function for an extended period of time. Although a power lead connected to the implanted device and extending outside the patient's body can be used to supply electrical power required to energize the device, any lead that passes through the skin increases the risk of infection if left in place for more than a few days. To avoid this problem, power can be supplied to an implanted medical device from an internal battery pack. However, any battery used for extended periods of time will eventually need to either be recharged or replaced. Replacing an internally implanted battery subjects the patient to further invasive surgery and is thus not desirable. Recharging a battery through directly connected leads again creates a risk of infection.
A solution to this problem is to recharge the battery by transcutaneously coupling power from an external source to an implanted receiver coil that is connected to the battery. In the prior art, an external transmitter coil is typically energized with an alternating current (AC), producing either a radio frequency (RF) signal or a varying magnetic flux that passes through the patient's skin and excites an implanted receiving coil. The electrical current that flows in the implanted receiving coil because of the RF signal that is received or as a result of the magnetic induction can be rectified and filtered for use in providing direct current (DC) power to the implanted device or in charging a battery pack that provides power to the implanted device. Alternatively, electrical current from the implanted coil may be directly applied to power the implanted medical device if the device will operate on AC at the frequency of the transmitted power. It should be noted that the implanted receiving coil and any related electronic circuitry for filtering and/or regulating the electrical current may be located at a different point in the patient's body from that at which the implanted medical device is disposed. The implanted medical device may be connected to the receiving coil and any associated electronic circuitry through a lead that passes through the patient's body between the two sites.
To provide sufficient power to energize an implanted medical device or recharge its battery, it may be necessary to energize the external transmitter coil with a relatively high current. The current flowing through the external transmitter coil and the power dissipated in the resistance of the coil can be sufficiently high to cause overheating of the external coil unless it is cooled using fans or some other cooling mechanism. Also, a potential shock hazard may exist, since several hundred volts may be applied to properly energize the external transmitter coil. Even if the housing of the external coil is adequately cooled by including one or more fans to move air through the housing, the heat generated in the transmitter that is carried by the fan driven air flow may blow over the patient's body and can cause the patient to feel uncomfortably warm.
Thus, it would clearly be preferable to provide an external transmitter that does not require cooling and which need not be energized with a substantial electrical current to induce the current required in an implanted coil. It would also be preferable if the external transmitter is relatively simple, more compact in size, lighter in weight, and lower in cost than conventional electromagnetic coil transmitters. It would also be desirable to provide a relatively simple mechanism for selectively adjusting the power delivered to an implanted coil from such an external transmitter.